Vibro-treating is a process for the surface improvement of metallic objects. For example, vibropolishing, also known as vibratory finishing, is commonly used to deburr, radius, descale, burnish, clean and brighten objects or parts of substantial hardness which require such surface improvement.
In vibropolishing, specially selected pellets, shot or tokens of media of a particular geometry and/or hardness are placed into an appropriately sized container. Objects requiring treatment are added to, or suspended within media contained within the container before the contents are vibrated. In the process of moving or vibrating the media relative to the object, or vice versa, the media rubs exposed areas of the object, causing a localised material removal from the tips of any outwardly extending asperities, so smoothing the surface of the object. As such, the media may also interact with internal features, such as holes or recesses, where active movement of media through the object is possible.
In use, such a rubbing or cutting action allows vibropolishing to produce an essentially smooth surface finish, bought about by what may be described as a substantial lapping action. Due to the fact that the bowl and object move as a substantially combined unit, fragile or delicate parts are supported by the media immediately surrounding the object, so making vibropolishing suitable for a wide range of delicate applications where improvement of surface finish is required.
However, there are a number of associated problems or known disadvantages with the presently available methods of vibropolishing which render the process unsuitable for certain applications. Accordingly, a fixed process, wherein items are held by a fixture and lowered into the media for treatment, only allows a specific number of objects to be attached to the fixture. Additionally, the fixed process and requires a large amount of processing time in order to mask objects, clamp objects and subsequently remove objects from the fixture, leading to reduced efficiency for a large number of small objects requiring treatment. For this reason, the fixed process is predominantly associated with the treatment of large aerospace objects.
In a separate surface improvement process, shot peening provides a means of cold working a surface of an object. This provides a compressive residual stress layer on the surface of a given object thus modifying the mechanical properties of the metals. The process of shot peening involves impacting a surface with metallic, glass, or ceramic shot with force sufficient to create a plastic deformation, thus imparting a layer of compressive residual stress.
In particular, vibropeening processes have been developed which provide an alternative to the combined processes of shot-peening and vibropolishing. In particular, vibropeening employs the inertia of heavier media to impart the required compressive residual stress onto the fan blade or aerofoil whilst concurrently smoothing the object and/or bringing about a material removal. Using the above method, the processing of a single object allows the ability to accurately control the application time and process parameters experienced by the object. This limits the output of a given machine and thus inhibits the cost performance of the vibropeening process. However, vibropeening also provides a number of disadvantages, including disprortionate vibropeening effect in different areas of the vibropeening container, leading to parts often receiving non-equivalent surface treatments.